FIG. 4 is a cross-sectional view of a typical prior art monolithic surface acoustic wave convolver comprising a piezoelectric layer 1, insulative layer 2, semiconductive epitaxial layer 3, semiconductive substrate 4, gate electrode 5, bottom electrode 6, comb-shaped electrodes 7, bias voltage source 8, inductance element L.sub.B and capacitor C.sub.B. Some other prior art devices do not include the insulative layer 2 and semiconductive epitaxial layer 3. In the most usual form, the piezoelectric layer is made from zinc oxide (ZnO) or aluminum nitride (AlN), the semiconductive epitaxial layer is made from silicon (Si), the insulative layer is made from silicon dioxide (SiO.sub.2), and the electrodes are made of aluminum (Al) or gold (Au) film.
The role of the device is to supply an output which is a convolution signal of two input signals. In FIG. 4, when input signals S.sub.1 and S.sub.2 are entered in respective comb-shaped electrodes 7 via input terminals IN.sub.1 and IN.sub.2, an output signal S.sub.OUT proportional to convolution signal of the input signals S.sub.1 and S.sub.2 is produced at an output terminal OUT through the gate electrode 5. The magnitude of the output S.sub.OUT varies with a bias voltage V.sub.B applied to the gate electrode 5. FIG. 5 shows a relationship between the convolution efficiency (symbolized by F.sub.T) and the bias voltage V.sub.B which relationship is expressed by: EQU S.sub.OUT =F.sub.T +S.sub.1 +S.sub.2 (1)
where respective values are in dBm.
The characteristic of FIG. 5 is of a device using an n-type semiconductor. When a p-type semiconductor is used, its curve is qualitatively inverted in sign of the voltage. As illustrated, the maximum efficiency is given by a value of the bias voltage which is normally several volts in the prior art devices.
With this value of the voltage, however, the semiconductor-insulator interface level or trapping at the insulator-piezoelectric interface or in the piezoelectric material would cause capture or creation of electrons or positive holes, and the time therefor would delay stabilization of the device.